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Buried pipes are not only subjected to mechanical actions (loads) but also environmental actions that cause the corrosion of pipes. Corrosion related defects would subsequently cause fracture of cast iron pipes. In the presence of corrosion pit, failure of a pipe can be attributed to two mechanisms: (i) the stresses in the pipe exceed the corresponding strength; or (ii) the stress intensity exceeds fracture toughness of the pipe. Based on these two failure modes, two limit state functions can be established as follows.

Steel pipe is manufactured by the pit, horizontal or centrifugal method. In the vertical pit method, a mold is made by ramming sand around a pattern and drying the mold in an oven. A core is inserted in the mold and molten iron is poured between the core and the mold. In the horizontal method, a machine is used to ram sand around horizontal molds that have core bars running through them. The molten iron is poured into the molds from multiple-lipped ladle designed to draw the iron from the bottom to eliminate the introduction of impurities. In the centrifugal method (Figure 3.4), sand-lined molds are used that are placed horizontally in centrifugal casting machines. While the mold revolves, an exact quantity of molten iron is introduced, which, by action of the speed of rotation, distributes itself on the walls of the mold to produce pipe within a few seconds.

Many cast iron pipes made towards the end of the nineteenth century are still in use; their walls were relatively thick and not always of uniform, ‘Spun’ grey iron pipes were formed by spinning in a mould and produced a denser iron with pipes of more uniform wall thickness; they comprise a large proportion of the distribution mains in many countries. Three classes of such pipes were available: B, C, and D for working pressures of 60, 90, and 120 m respectively; classes B and C were more widespread. Carbon is present in the iron matrix substantially in lamellar or flaky form; therefore, the pipes are brittle and relatively weak in tension and liable to fracture. The manufacture of grey iron pipes has been discontinued in most countries, except for the production of non-pressure drainage pipes.